The particularly strong conditions as to temperature and pressure that components in a gas turbine withstand make the material and the design of gas turbine components be of primary importance. Specifically, the blades of a gas turbine withstand strong operation conditions resulting in these blades being abraded with time. In order not to change the blades, which are very costly, every time they become abraded, it is known in the state of the art to use shroud devices that shield the blades, these devices being replaceable when needed in time.
Current shroud devices known in the state of the art consist of a metallic shroud having honeycombs embedded into it: typically, these honeycombs are composed of a thin metallic layer, having the problem that it oxidizes during the operation of the gas turbine, resulting in the shroud device being more brittle. For this reason, some solutions, as the one disclosed in U.S. Pat. No. 6,435,824 B2, replace the metallic honeycomb by a ceramic material, such as ceramic foam embedded in the metallic shroud. The main issue when using ceramic material (in foam or in any other way) is how to bind it to the metallic shroud configuring the shroud device, because of the thermal mismatch between ceramic materials and metallic materials, particularly super alloys used for gas turbine blades. The result is that, in these known solutions, high strain levels in the ceramic material occur during heating and/or cooling of the shroud device, ultimately resulting in the failure of the ceramic material and, therefore, in the failure of the shroud device.
Further solutions oriented to the reduction of strains due to the thermal mismatch of materials have been found and are known in the art: one of these solutions is a shroud device comprising a metallic shroud, a ceramic layer on top of it and a strain compliant layer between the metallic shroud and the ceramic layer. However, this strain compliant layer is ductile and has a limited strength: thus, for applications where a high level of shear (strain) stresses are applied to both the ceramic layer and the strain compliant layer, a compromise has to be found between the strain (shear) compliance and the strength, which is not easy to achieve.
Some other known solutions for attaching a ceramic material to a metal layer are brazing or, in case of a ceramic foam being used, by infiltration, as disclosed in U.S. Pat. No. 6,435,824 B2. However, all these known solutions present the drawback that any failure of the ceramic material requires the exchange of the whole shroud device, which is costly and time consuming. Another solution known is to fix the metallic layer and the ceramic layer by mechanical clamping: however, this solution results in stress accumulated in the ceramic layer, which can lead to the failure of it and, thus, of the complete shroud device.
The present invention is directed towards solving the above-mentioned drawbacks in the prior art.